1. Field of the Invention
This invention relates to a method of making a rare earth magnet having a principal phase composed of a compound represented by the formula R.sub.2 Fe.sub.14 B, where R is a rare earth component consisting mainly of one or both of neodymium and praseodymium. The magnets according to this invention are expected to find a wide scope of use in various kinds of actuators including small motors, since they could be inexpensive and yet exhibit a high level of performance.
2. Description of the Prior Art
It is possible to produce a quenched ribbon having excellent magnetic properties if a molten alloy containing a rare earth element (R), iron (Fe), as a typical transition metal element, and boron (B), substantially in the ratio of 2:14:1 is very rapidly quenched by a liquid quenching method employing, for example, a single roll (see Japanese Patent Applications laid open under Nos. 59-64739 and 60-9852). More specifically, a flaky ribbon having a thickness of about 30 microns can be produced if a molten Nd-Fe-B alloy is ejected on the surface of a rotating copper roll. It is known that the ribbon may have an amorphous structure, or a finely crystalline structure with a grain size of 0.01 to 0.5 micron, depending on the cooling rate which is employed. The ribbon exhibits a high intrinsic coercive force when it has a grain size of about 0.05 micron.
A powder obtained by crushing a rapidly quenched ribbon of the Nd-Fe-B alloy can be shaped into a body retaining its excellent magnetic properties and having a density close to that of the alloy. A hot pressed body is slightly magnetically anisotropic, as is taught in Japanese Patent Application laid open under No. 60-10042, which corresponds to U.S. patent application Ser. No. 520,170 filed on Aug. 4, 1983, and as is also reported by R. W. Lee in his paper entitled "Hot-Pressed Neodymium-Iron-Boron Magnets", Applied Physics Letters, Vol. 46, No. 8, Apr. 15, 1985, pp. 790-791. When the powder of the Nd-Fe-B alloy is hot pressed, a plastic flow occurs in the hot pressed body and the resulting rotation of crystals tends to produce the axis of easy magnetization (c-axis) parallel to the direction of pressing. It has been reported that a residual magnetic flux density of about 8 kG is achieved in the direction of pressing.
According to R. W. Lee, a higher residual magnetic flux density can be achieved if a hot pressed product of the Nd-Fe-B alloy having a density close to that of the alloy is subjected to die upsetting to undergo plastic deformation. It has been reported that a residual magnetic flux density of 8 to 13 kG is achieved, depending on the degree of die upsetting and the composition of the alloy (e.g., Y. Nozawa et al., J. Appl. Phys., Vol. 64, No. 10, Nov. 15, 1988, pp. 5285-5289).
Drawbacks, however, exist in a magnet produced as a hot pressed product of the powder prepared from a rapidly quenched ribbon of the Nd-Fe-B alloy, as will hereinafter be pointed out.
The anisotropic sintered magnets of a samarium-cobalt alloy (SmCo.sub.5 or Sm.sub.2 Co.sub.17) which are reliably used in small motors, etc. exhibit a residual magnetic flux density of 8.5 to 10.5 kG. As samarium is expensive, however, these magnets are partly being replaced by sintered magnets of the Nd-Fe-B alloy, since neodymium is less expensive. No magnet obtained as a hot pressed product of the powder prepared from a rapidly quenched ribbon of the Nd-Fe-B alloy can, however, be a satisfactory substitute for any sintered Sm-Co magnet, since it exhibits a residual magnetic flux density of only about 8 kG, as hereinabove stated. A hot pressed and die upset product can be a good substitute, as far as its magnetic properties are concerned, but the addition of the die upsetting process presents other problems. The die upsetting process not only adds to the time and labor which are required for producing a magnet, but also renders it difficult to make a product of the desired shape. Accordingly, it cannot necessarily be said to be less expensive than a sintered Sm--Co magnet, though neodymium itself is less expensive. For making an inexpensive rare earth magnet, therefore, it is desirable to employ a method which enables mass production and also eliminates the necessity for any final cutting or grinding process, or the so-called "near-net shaping" process.